An EMI gasket is an electrically conductive interface material that is used to connect an electrically conductive shield with a corresponding section of an electrical ground, such as a ground trace of a printed circuit board (PCB). The ground trace typically encircles components that either emit electromagnetic interference or are sensitive to such interference. Preferably, an EMI gasket should be highly electrically conductive and conformal under pressure. Once assembled, the gasket material mechanically (and thus, electrically) fills the gaps between the shield and ground trace, forming a Faraday Cage around the components, thereby shielding them from EMI.
Since the mating surfaces of both a shield and PCB are not perfectly parallel and flat (for tolerance reasons), gaps are created at the interface, which can leak EMI. Gaps can also result from flexing. An EMI gasket must be compressible so that it fills in these gaps, thereby consistently maintaining electrical contact between the shield and ground trace over the intended design range of tolerances and flexing. As cellular phones and other devices become smaller and lighter, the materials they are made from become more susceptible to this flexing. As market pressures continue to push pricing downwards, designers have been forced to loosen flatness specifications on shield and PCB parts, and decrease the number of typical screw fasteners (replacing them with snap features). Typically, previous shield and PCB designs have required approximately 0.2 to 0.3 mm of conformability (compression) out of an EMI gasket material. Today's designs are pushing that requirement more towards 0.4 to 0.6 mm, yet with less available force. These trends place a higher demand upon the EMI gasket, forcing it to be more compressible under lower forces (i.e. softer), yet still delivering proper electrical performance. This is pushing the very limits of EMI gasket technology.
Presently, EMI gaskets are almost exclusively installed directly onto the shield, as opposed to the PCB, and often must have a customized pattern for each application. These EMI gaskets are typically comprised of conductive particles in a binder, such as silicone. They are typically dense and difficult to compress, since they necessarily rely on the conductive particles to contact each other. Stamped metal parts with formed compressible spring-like ‘fingers’ are another type of gasket material, but these are known to have high-frequency performance issues due to the ‘finger’ spacing, as well as electrical reliability issues over time and exposure to elevated temperature and humidity. The present manufacturing techniques for installing such EMI gaskets include the following: dispensing a conductive particle-filled paste or liquid material directly onto the shield's surface and curing the dispensed material in-situ; die-cutting a conductive sheet material having an adhesive backer and then transferring, positioning and adhering the dimensioned material directly to the shield's surface; or mechanically fastening a conductive spring-like metal material to the shield's surface.
Conductive EMI gasket “ropes” have been developed to address the issue of softness. These are typically extruded tubes that are either somewhat homogeneously comprised of conductive particles in an elastomeric binder, or so-called “dual-extrusion” types which are comprised of a dielectric, elastomeric inner core and an electrically conductive outer layer, typically conductive particles in an elastomeric binder. Silicone is often used for the binder and/or core, due to its mechanical resiliency and high temperature resistance. These “ropes” can offer high compressibility with low forces, owing to their tube-like shape and hollow center (compression hole). Since they are extruded, however, they cannot be customized to complex, customized shapes, which limits their flexibility particularly in complicated handset designs. Using separate individual pieces of such a material can add design flexibility, but requires the use of an adhesive and complex peel-and-stick automation, which is cumbersome, difficult and often unreliable. Alternatively, a groove in the shield can be used to retain the tube or tube pieces, but this consumes significant space in a design, which is at a premium in handsets.
As is well understood by those skilled in the art, SMT (Surface Mount Technology) machines typically utilize a vacuum head on the end of a high-speed pick-and-place system to install tape-and-reel-fed PCB components onto surface-mount pads on a PCB. These pads are usually pre-screened with solder-paste (or conductive adhesive) and then sent through a reflow process (such as infrared-IR, vapor-phase, or convection) to melt the solder joints (or cure the adhesive), thereby forming an electrical and mechanical connection. As stated before, EMI gaskets are generally installed directly onto the shielding cover, which mates to a matching ground trace on the PCB. However, there is an SMT-compatible EMI gasket assembly that can be placed directly onto the PCB, and soldered (or bonded) to the ground trace. This gasket assembly can also be soldered or bonded to the shielding cover, as well, for additional flexibility. This type of gasket assembly is available under the trade name of GORE-SHIELD® SMT EMI gaskets. Gore's U.S. Pat. Nos. 6,235,986 and 6,255,581 are incorporated herein by reference. A typical SMT-compatible gasket assembly solution might be made up of a multitude of individual, gasket assemblies combined to form a more-or-less continuous gasket design, or just a few discrete assemblies used in selected areas, depending upon the shielding performance required. The benefits of this approach are well known, but include allowing for standard assembly sizes, design flexibility, speed of incorporating design changes, and providing a cost-effective way to add consistent, discrete point grounding to an EMI shielding scheme. These SMT-compatible EMI gasket assemblies have been particle-filled, which suffer from some of the same compressibility issues as dispensed type gaskets. What is needed for today's designs, however, is an SMT-compatible gasket assembly that is softer than existing products. This would allow for lighter, thinner, less expensive covers and housings with a larger range of tolerances, and allow for larger ranges of deflection, as described previously.